FEAT: Add availabilityService (#81)

* auto and fallback work

* test fixes

* fixes

* Show model dialog even if there's no more fallback option

* fix tests

* fix failing test

* disable showInDialog for model in settings

* revert package-lock.json

* remove dup line

---------

Co-authored-by: Sehoon Shon <sshon@google.com>

packages/cli/src/zed-integration/zedIntegration.ts

@@ -257,7 +257,7 @@ export class Session {
       try {
         const model = getEffectiveModel(
           this.config.getModel(),
-          this.config.isInFallbackMode(),
+          this.config.getPreviewFeatures(),
         );
         const responseStream = await chat.sendMessageStream(
           { model },